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BACKGROUND The purpose of this research was to explore the possibility of co‐applying pyrethroids (agonists of voltage‐sensitive sodium channels) with potassium channel blockers in order to potentiate the neurological effects of pyrethroids on Anopheles gambiae. We hypothesized that the toxicity of pyrethroids caused by persistent sodium currents would be augmented by blockage of outward potassium current flow, which normally repolarizes the membrane potential during a nerve membrane action potential. RESULTS: Topical treatments with LD10s (10% mortality doses) of synergists were given with pyrethroids. 2S‐65465 (2S) showed the best synergism of permethrin (8.6‐fold) and deltamethrin (7.2‐fold), whereas piperonyl butoxide and 4‐aminopyridine only showed 2.2‐ to 3.4‐fold synergism with these pyrethroids. In electrophysiological recordings of Periplaneta americana giant axons, 2S (10 μm) and 4‐AP (30 μm) caused multiple spikes after a single stimulation. Permethrin at 10 μm showed significant summating depolarization (4.5 ± 1.1 mV) after a train of ten stimuli were applied at 5 Hz, and deltamethrin at 0.03 μm showed significant membrane depolarization of 2.9 ± 0.4 mV without stimuli. 2S at 0.3 μm and 4‐AP at 1–3 μm significantly synergized the effects of 3 μm permethrin and 0.01 μm deltamethrin. CONCLUSIONS: Co‐application of potassium channel blockers 2S and 4‐AP with pyrethroids can synergize the mosquitocidal activities on An. gambiae, and these activities are correlated with synergistic effects at the level of the nerve membrane. If deployed in the field, this approach can potentially reduce the amount of chemicals needed for effective control of mosquitoes. © 2020 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

BACKGROUND The compound 2‐((4‐ethyl‐5‐(pyridin‐3‐yl)‐4H‐1,2,4‐triazol‐3‐yl)thio)‐N‐(4‐ethylphenyl) acetamide (VUAA1) is reported to be an odorant receptor co‐receptor (Orco) agonist in insects with potential use as an insect repellent. For this study, the biological activity of VUAA1 was investigated in several bioassays with Aedes aegypti, including adult contact, spatial repellency, and larval repellency assays, as well as topical, injection, and feeding toxicity assays. Neurophysiological action was further explored by analysis of fruit fly central nervous system firing, cockroach axon recordings, patch clamp analysis of Kv2 potassium channel, and acetylcholinesterase inhibition studies. Finally, the metabolic impact on the toxicity of VUAA1 was explored by applying it in combination with established metabolic synergists. RESULTS: In repellency and bite protection screens, VUAA1 showed little activity against adult mosquitoes, apparently due to its low volatility, since its effectiveness was increased by heating or mixing with transfluthrin acid and citronella oil. It did produce measurable repellency of mosquito larvae that was more potent than N,N‐diethyl‐m‐toluamide (DEET). Overall, VUAA1 showed low acute toxicity to both insects and mice, and it was weakly synergized by triphenyl phosphate. There was no observed cross‐resistance in a pyrethroid‐resistant strain of Anopheles gambiae. VUAA1 showed a two‐phase effect on the central nervous system, with neuroexcitation at 1 μmol L‐1 and an inhibitory effect at 100 μmol L‐1 that may relate to block of Kv2 potassium channels. CONCLUSIONS: VUAA1 presented low toxicity, similar to other insect repellents. Its limited solubility, low volatility, and resulting poor adult repellency without additional adjuvants may restrict the utility of VUAA1 in typical public health applications. © 2020 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

BACKGROUND: Insecticide resistance in the malaria mosquito Anopheles gambiae is well documented, and widespread agricultural use of pyrethroids may exacerbate development of resistance when pyrethroids are used in vector control. We have developed carbamate anticholinesterases that possess a high degree of An. gambiae:human selectivity for enzyme inhibition. The purpose of this study was to assess the spectrum of activity of these carbamates against other mosquitoes and agricultural pests. RESULTS: Experimental carbamates were potent inhibitors of mosquito acetylcholinesterases, with IC50 values in the nanomolar range. Similar potencies were observed for Musca domestica and Drosophila melanogaster enzymes. Although meta-substituted carbamates were potent inhibitors, two ortho-substituted carbamates displayed poor enzyme inhibition (IC50 ≥10-6 M) in honey bee (Apis mellifera), Asian citrus psyllid (Diaphorina citri) and lepidopteran agricultural pests (Plutella xylostella and Ostrinia nubilalis). Enzyme inhibition results were confirmed by toxicity studies in caterpillars, where the new carbamates were 2-3-fold less toxic than propoxur and up to tenfold less active than bendiocarb, indicating little utility of these compounds for crop protection. CONCLUSION: The experimental carbamates were broadly active against mosquito species but not agricultural pests, which should mitigate selection for mosquito insecticide resistance by reducing agricultural uses of these compounds. [ABSTRACT FROM AUTHOR]

BACKGROUND: Insecticide resistance in the malaria mosquito Anopheles gambiae is well documented, and widespread agricultural use of pyrethroids may exacerbate development of resistance when pyrethroids are used in vector control. We have developed carbamate anticholinesterases that possess a high degree of An. gambiae:human selectivity for enzyme inhibition. The purpose of this study was to assess the spectrum of activity of these carbamates against other mosquitoes and agricultural pests. RESULTS: Experimental carbamates were potent inhibitors of mosquito acetylcholinesterases, with IC50 values in the nanomolar range. Similar potencieswere observed for Musca domestica and Drosophila melanogaster enzymes. Although meta-substituted carbamates were potent inhibitors, two ortho-substituted carbamates displayed poor enzymeinhibition (IC50 ≥10-6 M)inhoney bee (Apis mellifera), Asian citrus psyllid (Diaphorina citri) and lepidopteran agricultural pests (Plutella xylostella and Ostrinia nubilalis). Enzyme inhibition results were confirmed by toxicity studies in caterpillars, where the new carbamates were 2-3-fold less toxic than propoxur and up to tenfold less active than bendiocarb, indicating little utility of these compounds for crop protection. CONCLUSION: The experimental carbamates were broadly active against mosquito species but not agricultural pests, which should mitigate selection for mosquito insecticide resistance by reducing agricultural uses of these compounds. [ABSTRACT FROM AUTHOR]

We explored the potential of two sodium channel activators, veratrine and aconitine, as both insecticides and synergists of natural pyrethrins (NP) on Aedes aegypti adults and larvae. Aconitine was more toxic than veratrine, with an LD 50 of 157 ng/mg compared to 376 ng/mg, on the pyrethroid-susceptible Orlando strain, but only aconitine showed significant resistance in the pyrethroid-resistant Puerto Rico strain (RR = 14.6 in topical application and 8.8 in larval bioassay). When applied in mixtures with piperonyl butoxide (PBO) and NP, large synergism values were obtained on the Orlando strain. Aconitine + PBO mixture synergized NP 21.8-fold via topical adult application and 10.2-fold in larval bioassays, whereas veratrine + PBO synergized NP 5.3-fold via topical application and 30.5-fold in larval bioassays. Less synergism of NP was observed on the resistant Puerto Rico strain, with acontine + PBO synergizing NP only 4.1-fold in topical application (8-fold in larval bioassays) and veratrine + PBO synergizing NP 9.5-fold in topical application (13.3-fold in larval bioassays). When alkaloids were applied directly to the mosquito larval nervous system, veratrine was nearly equipotent on both strains, while aconitine was less active on pyrethroid-resistant nerve preparations (no block at 10 μM compared to block at 1 μM on the susceptible strain). The nerve blocking effect of NP was significantly synergized by both compounds on the pyrethroid-susceptible strain by about 10-fold, however only veratrine synergized NP block on the pyrethroid-resistant strain, also showing 10-fold synergism). These results highlight the potential of site II sodium channel activators as insecticides and their ability to synergize pyrethroids, which may extend the commercial lifetime of these chemistries so essential to public health vector control. [Display omitted] • Veratrine and aconitine are capable of synergizing natural pyrethrins (NP) on adult and larvae of Aedes aegypti. • Both alkaloids synergized NP on pyrethroid-susceptible strain; only veratrine synergized NP on the resistant strain. • Veratrine was more potent than aconitine (by itself and as a synergist of NP) on cockroach nerve cords. • Co-application with PBO increased synergism multiplicatively and suggested that synergism occurred via novel mechanism. [ABSTRACT FROM AUTHOR]

Acetylcholinesterase (AChE) is a proven target for control of the malaria mosquito (Anopheles gambiae). Unfortunately, a single amino acid mutation (G119S) in An. gambiae AChE-1 (AgAChE) confers resistance to the AChE inhibitors currently approved by the World Health Organization for indoor residual spraying. In this report, we describe several carbamate inhibitors that potently inhibit G119S AgAChE and that are contact-toxic to carbamate-resistant An. gambiae. PCR-RFLP analysis was used to confirm that carbamate-susceptible G3 and carbamate-resistant Akron strains of An. gambiae carry wild-type (WT) and G119S AChE, respectively. G119S AgAChE was expressed and purified for the first time, and was shown to have only 3% of the turnover number (kcat) of the WT enzyme. Twelve carbamates were then assayed for inhibition of these enzymes. High resistance ratios (>2,500-fold) were observed for carbamates bearing a benzene ring core, consistent with the carbamate-resistant phenotype of the G119S enzyme. Interestingly, resistance ratios for two oxime methylcarbamates, and for five pyrazol-4-yl methylcarbamates were found to be much lower (4- to 65-fold). The toxicities of these carbamates to live G3 and Akron strain An. gambiae were determined. As expected from the enzyme resistance ratios, carbamates bearing a benzene ring core showed low toxicity to Akron strain An. gambiae (LC50>5,000 µg/mL). However, one oxime methylcarbamate (aldicarb) and five pyrazol-4-yl methylcarbamates (4a-e) showed good to excellent toxicity to the Akron strain (LC50 = 32-650 µg/mL). These results suggest that appropriately functionalized ''small-core'' carbamates could function as a resistance-breaking anticholinesterase insecticides against the malaria mosquito. [ABSTRACT FROM AUTHOR]

Anopheles gambiae Giles (Diptera: Culicidae) is the most prolific malaria vector in sub-Saharan Africa, where widespread insecticide resistance has been reported. An. gambiae laboratory strains are commonly used to study the basic biology of this important mosquito vector, and also in new insecticide discovery programs, where insecticide-susceptible and -resistant strains are often used to screen new molecules for potency and cross-resistance, respectively. This study investigated the toxicity of permethrin, a Type-I pyrethroid insecticide, and etofenprox, a non-ester containing pyrethroid insecticide, against An. gambiae at three life stages. This characterization was performed with susceptible (G3; MRA-112) and resistant (Akdr; MRA-1280) An. gambiae strains; the Akdr strain is known to contain the L1014F mutation in the voltage-sensitive sodium channel. Surprisingly, etofenprox displays a lower level of resistance than permethrin against all stages of mosquitoes, except in a headless larval paralysis assay designed to minimize penetration factors. In first-instar An. gambiae larvae, permethrin had significant resistance, determined by the resistance ratio (RR50 = 5), but etofenprox was not significantly different (RR50 = 3.4) from the wild-type strain. Fourth-instar larvae displayed the highest level of resistance for permethrin (RR50 = 108) and etofenprox (RR50 = 35). Permethrin (PC50 = 2 ppb) and etofenprox (PC50 = 9 ppb) resulted in headless larval paralysis (5-h), but resistance, albeit lower, was still present for permethrin (RR50 = 5) and etofenprox (RR50 = 6.9). In adult female mosquitoes, permethrin displayed higher resistance (RR50 = 14) compared to etofenprox (RR50 = 4.3). The level of etofenprox resistance was different from that previously reported for a similar Akron An. gambiae laboratory strain (MRA-913). The chemical synergists piperonyl butoxide (PBO) and diethyl maleate (DEM) were able to synergize permethrin, but not etofenprox in the resistant strain (Akdr). In conclusion, multiple mechanisms are likely involved in pyrethroid resistance, but resistance profiles are dependent upon selection. Etofenprox is an effective insecticide against An. gambiae in the lab but will likely suffer from resistance in the field. [ABSTRACT FROM AUTHOR]

The widespread deployment of insecticide-treated bednets (ITNs) in sub-Saharan Africa has led to a dramatic decline in malaria mortality. However, wide-spread and growing resistance of Anopheles gambiae mosquitoes to the pyrethroid class of voltage-gated Na+ channel modulators used on these nets jeopardizes this achievement, and has prompted the search for suitable insecticidal AChE inhibitors to replace pyrethroids. Such compounds would have three favorable characteristics: excellent contact toxicity towards susceptible adult An. gambiae, good contact toxicity to those that bear the G119S resistance mutation of AChE, and very weak inhibition of human AChE.¹ We will review our work on the development of aromatic and heterocyclic core methyl and dimethylcarbamate AChE inhibitors,2 and including both enzymatic inhibition potencies and mosquito contact toxicities. Finally, the inhibition selectivities of particular compounds will be rationalized in the context of our recently obtained high resolution X-ray structures of G119S An. gambiae AChE.³. [ABSTRACT FROM AUTHOR]

The goal of this research was to express receptors and ion channels in hormone-treated insect cell lines. Treatment of Anopheles gambiae Sua1B cells with 20-hydroxyecdysone showed an inhibition of cell growth over a time course of three days, with no change in cellular morphology. The effect of 20-hydroxyecdysone was enhanced in the presence of the potassium channel blocker 4-aminopyridine, but not tetraethylammonium. Concentration-response curves of 4-aminopyridine in the presence of 42 μM (1 mg/ml) 20-hydroxyecdysone showed similar IC 50 values (6–10 μM) across 3 day exposures. Whole cell patch clamp confirmed the expression of delayed-rectifier (K v2 ) potassium channels in hormone-supplemented Sua1B cells, whereas untreated Sua1B cells showed no evidence of Kv2 expression. The hormone-induced expression of K v2 channels occurred in as little as 4 h after treatment, but were not observed after 24 h of exposure to 20-hydroxyecdysone, suggesting they played a role in cell death. The expressed channels had current-voltage relationships diagnostic for the K v2 subtype, and were inhibited with an IC 50 = 13 mM of tetraethylammonium. Overall, these parameters were similar to Anopheles gambiae Kv2 potassium channels expressed in HEK-293 cells. The induced presence of ion channels (and possibly receptors) in these cells has potential utility for high throughput screening and basic neuroscience research. [ABSTRACT FROM AUTHOR]

Liriodenine is a biologically active plant alkaloid with multiple effects on mammals, fungi, and bacteria, but has never been evaluated for insecticidal activity. Accordingly, liriodenine was applied topically in ethanolic solutions to adult female Anopheles gambiae , and found to be mildly toxic. Its lethality was synergized in mixtures with dimethyl sulfoxide and piperonyl butoxide. Recordings from the ventral nerve cord of larval Drosophila melanogaster showed that liriodenine was neuroexcitatory and reversed the inhibitory effect of 1 mM GABA at effective concentrations of 20–30 μM. GABA antagonism on the larval nervous system was equally expressed on both susceptible and cyclodiene-resistant rdl preparations. Acutely isolated neurons from Periplaneta americana were studied under patch clamp and inhibition of GABA-induced currents with an IC 50 value of about 1 μM were observed. In contrast, bicuculline did not reverse the effects of GABA on cockroach neurons, as expected. In silico molecular models suggested reasonable structural concordance of liriodenine and bicuculline and isosteric hydrogen bond acceptor sites. This study is the first assessing of the toxicology of liriodenine on insects and implicates the GABA receptor as one likely neuronal target, where liriodenine might be considered an active chemical analog of bicuculline. [Display omitted] • Liriodenine had low insecticidal activity, synergized by DMSO & PBO. • It excited Drosophila ventral cord and reversed GABA blocking effects. • It blocked GABA A receptors in patch clamped cockroach neurons. • Molecular modeling shows some similarity in structure to bicuculline. • The results suggest it could be an active analog of bicuculline in insects. [ABSTRACT FROM AUTHOR]

Abstract The present study focused on the toxicity of the aphid anti-feedant flonicamid and its main metabolite, 4-trifluoromethylnicotinamide (TFNA-AM) to Aedes aegypti and Anopheles gambiae mosquitoes. The compounds were toxic to both species via topical application, resulting in un-coordinated locomotion and leg splaying, with a favorable An. gambiae LD 50 value of 35 ng/mg for TFNA-AM, but no significant lethality to Ae. aegypti at 10 μg/female. There was mild cross resistance in the Akron -kdr (Akdr) strain of An. gambiae. Both compounds were non-toxic to intact larvae (LC 50 > 300 ppm); however, headless Ae. aegypti larvae displayed spastic paralysis, with PC 50 values of 2–4 ppm, indicating that the cuticle is a significant barrier to penetration. TFNA-AM showed low mammalian toxicity, with an LD 50 of >2000 mg/kg in mice. Electrophysiological experiments showed larval Aedes muscle depolarization and Kv2 channel blocking activity that required near mM concentrations, suggesting that this potassium channel is not the main target for flonicamid nor its metabolite. However, TFNA-AM was a potent blocker of evoked body wall sensory discharge in dipteran larvae, suggesting that some component of the chordotonal organ system may be involved in its toxicity. Finally, flonicamid and TFNA-AM showed about 2-fold synergism of permethrin toxicity against An. gambiae adult females whose mechanism should become more clear once the mode of action of these compounds is better defined. Graphical abstract Unlabelled Image Highlights • Flonicamid and its metabolite TFNA-AM had toxic and behavior-modifying properties. • Cross resistance in the Akdr strain of Anopheles gambiae was low. • The compounds depolarized Aedes muscle and blocked the Kv2 channel at near mM concentrations. • TFNA-AM (2 μM), but not flonicamid, blocked firing of sensory nerves. • Primary site of action is chordotonal organs, with some effects on potassium channels. [ABSTRACT FROM AUTHOR]

Abstract The widespread emergence of pyrethroid-resistant Anopheles gambiae has intensified the need to find new contact mosquitocides for indoor residual spraying and insecticide treated nets. With the goal of developing new species-selective and resistance-breaking acetylcholinesterase (AChE)-inhibiting mosquitocides, in this report we revisit the effects of carbamate substitution on aryl carbamates, and variation of the 1-alkyl group on pyrazol-4-yl methylcarbamates. Compared to aryl methylcarbamates, aryl dimethylcarbamates were found to have lower selectivity for An. gambiae AChE (Ag AChE) over human AChE (hAChE), but improved tarsal contact toxicity to G3 strain An. gambiae. Molecular modeling studies suggest the lower species-selectivity of the aryl dimethylcarbamates can be attributed to a less flexible acyl pocket in Ag AChE relative to hAChE. The improved tarsal contact toxicity of the aryl dimethylcarbamates relative to the corresponding methylcarbamates is attributed to a range of complementary phenomena. With respect to the pyrazol-4-yl methylcarbamates, the previously observed low An. gambiae -selectivity of compounds bearing α-branched 1-alkyl groups was improved by employing β- and γ-branched 1-alkyl groups. Compounds 22a (cyclopentylmethyl), 21a (cyclobutylmethyl), and 26a (3-methylbutyl) offer 250-fold, 120-fold, and 96-fold selectivity, respectively, for inhibition of Ag AChE vs. hAChE. Molecular modeling studies suggests the high species-selectivity of these compounds can be attributed to the greater mobility of the W84 sidechain in the choline-binding site of Ag AChE, compared to that of W86 in hAChE. Compound 26a has reasonable contact toxicity to G3 strain An. gambiae (LC 50 = 269 μg/mL) and low cross-resistance to Akron strain (LC 50 = 948 μg/mL), which bears the G119S resistance mutation. Graphical abstract Unlabelled Image Highlights • Aryl and pyrazol-4-yl carbamates were explored as An. gambiae and human AChE inhibitors. • Dimethylcarbamates have lower inhibition selectivity than methylcarbamates. • Beta- and gamma-branched alkyl groups confer An. gambiae AChE inhibition selectivity. • Comparison of hAChE and G119S AgAChE crystal structures explains these selectivities. [ABSTRACT FROM AUTHOR]

New public health insecticides are urgently required to prevent the spread of vector-borne disease. With the goal of identifying new K + -channel-directed mosquitocides, analogs of the RH-5849 family of diacyl t -butylhydrazines were synthesized and tested for topical toxicity against adult Anopheles gambiae , the African vector of malaria. In total, 80 N ′-monoacyl and N , N ′-diacyl derivatives of benzyl- and arylhydrazines were prepared. Three compounds ( 2bo , 2kb , 3ab ) were identified that were more toxic than RH-5849 and RH-1266. The potencies of these compounds to block K + currents in An. gambiae and human Kv2.1 channels were assessed to address their possible mechanism of mosquitocidal action. Selectivity for inhibition of An. gambiae Kv2.1 vs human Kv2.1 did not exceed 3-fold. Furthermore, no correlation was seen between the potency of insecticidal action and K + channel blocking potency. These observations, combined with the minimal knockdown seen with 2bo near its LD 50 value, suggests a mode of action outside of the nervous system. [ABSTRACT FROM AUTHOR]

Potential targets for new vector control insecticides are nerve and muscle potassium channels. In this study, the activities of known potassium channel blockers (4-aminopyridine, quinidine, and tetraethylammonium) and the insecticide propoxur were compared to three experimental catechols and several other compounds against Anopheles gambiae and Aedes aegypti mosquitoes. Experimental catechol 1 was the most toxic experimental compound in all of the mortality assays conducted, but was at least 100-fold and 39-fold less toxic than propoxur against Ae. aegypti and An. gambiae , respectively. Injection treatment and synergist (piperonyl butoxide) bioassays found that catechol toxicity was not unduly impacted by cuticular transport or oxidative metabolism. Electrophysiological studies showed a decrease in amplitude of evoked muscle contractions, along with an increase in twitch duration at concentrations that increased basal muscle tension (mM). High concentration effects on basal muscle tension were matched by complete depolarization of the muscle membrane potential. Effects on muscle physiology and blockage of Kv2.1 potassium channels in patch clamp experiments were generally consistent with in vivo toxicity, except for 4-aminopyridine, which suggest the involvement of other potassium channel subtypes. Extensive melanization of Anopheles larvae, but not Aedes larvae, occurred from exposure to catechol compounds. Interaction with the phenol oxidase system within insects may be the cause of this melanization, but any contribution to toxicity requires further investigation. [ABSTRACT FROM AUTHOR]

Malaria is a devastating disease in sub-Saharan Africa, and current vector control measures are threatened by emerging resistance mechanisms. With the goal of developing new, selective, resistance-breaking insecticides we explored α-fluorinated methyl ketones as reversible covalent inhibitors of Anopheles gambiae acetylcholinesterase ( Ag AChE). Trifluoromethyl ketones 5 demonstrated remarkable volatility in microtiter plate assays, but 5c , e – h exhibited potent (1–100 nM) inhibition of wild type (WT) Ag AChE and weak inhibition of resistant mutant G119S mutant Ag AChE. Fluoromethyl ketones 10c – i exhibited submicromolar to micromolar inhibition of WT Ag AChE, but again only weakly inhibited G119S Ag AChE. Interestingly, difluoromethyl ketone inhibitors 9c and 9g had single digit nanomolar inhibition of WT Ag AChE, and 9g had excellent potency against G119S Ag AChE. Approach to steady-state inhibition was quite slow, but after 23 h incubation an IC 50 value of 25.1 ± 1.2 nM was measured. We attribute the slow, tight-binding G119S Ag AChE inhibition of 9g to a balance of steric size and electrophilicity. However, toxicities of 5g , 9g , and 10g to adult A. gambiae in tarsal contact, fumigation, and injection assays were lower than expected based on WT Ag AChE inhibition potency and volatility. Potential toxicity-limiting factors are discussed. [ABSTRACT FROM AUTHOR]

Insecticide resistance in the malaria vector, Anopheles gambiae , is a serious problem, epitomized by the multi-resistant Akron strain, originally isolated in the country of Benin. Here we report resistance in this strain to pyrethroids and DDT (13-fold to 35-fold compared to the susceptible G3 strain), but surprisingly little resistance to etofenprox, a compound sometimes described as a “pseudo-pyrethroid.” There was also strong resistance to topically-applied commercial carbamates (45-fold to 81-fold), except for the oximes aldicarb and methomyl. Biochemical assays showed enhanced cytochrome P450 monooxygenase and carboxylesterase activity, but not that of glutathione- S -transferase. A series of substituted α,α,α,-trifluoroacetophenone oxime methylcarbamates were evaluated for enzyme inhibition potency and toxicity against G3 and Akron mosquitoes. The compound bearing an unsubstituted phenyl ring showed the greatest toxicity to mosquitoes of both strains. Low cross resistance in Akron was retained by all analogs in the series. Kinetic analysis of acetylcholinesterase activity and its inhibition by insecticides in the G3 strain showed inactivation rate constants greater than that of propoxur, and against Akron enzyme inactivation rate constants similar to that of aldicarb. However, inactivation rate constants against recombinant human AChE were essentially identical to that of the G3 strain. Thus, the acetophenone oxime carbamates described here, though potent insecticides that control resistant Akron mosquitoes, require further structural modification to attain acceptable selectivity and human safety. [ABSTRACT FROM AUTHOR]

To identify potential selective and resistance-breaking mosquitocides against the African malaria vector Anopheles gambiae , we investigated the acetylcholinesterase (AChE) inhibitory and mosquitocidal properties of isoxazol-3-yl dimethylcarbamates ( 15 ), and the corresponding 3-oxoisoxazole-2(3 H )-dimethylcarboxamide isomers ( 14 ). In both series, compounds were found with excellent contact toxicity to wild-type susceptible (G3) strain and multiply resistant (Akron) strain mosquitoes that carry the G119S resistance mutation of AChE. Compounds possessing good to excellent toxicity to Akron strain mosquitoes inhibit the G119S mutant of An. gambiae AChE ( Ag AChE) with k i values at least 10- to 600-fold higher than that of propoxur, a compound that does not kill Akron mosquitoes at the highest concentration tested. On average, inactivation of WT Ag AChE by dimethylcarboxamides 14 was 10–20 fold faster than that of the corresponding isoxazol-3-yl dimethylcarbamates 15 . X-ray crystallography of dimethylcarboxamide 14d provided insight into that reactivity, a finding that may explain the inhibitory power of structurally-related inhibitors of hormone-sensitive lipase. Finally, human/ An. gambiae AChE inhibition selectivities of these compounds were low, suggesting the need for additional structural modification. [ABSTRACT FROM AUTHOR]

Highlights: [•] Carbamates exhibit selectivity for Anopheles gambiae acetylcholinesterase (AChE) over human. [•] Carbamates were tested on AChE from non-target species and exhibited up to 582-fold selectivity. [•] In vivo assay of D. magna confirmed low or acceptable toxicity to aquatic invertebrates. [•] Neurotoxic esterase was unaffected by experimental carbamates (up to 1mM). [•] Experimental carbamates possess more favorable safety profiles than commercial carbamates. [ABSTRACT FROM AUTHOR]

Abstract: New carbamates that are highly selective for inhibition of Anopheles gambiae acetylcholinesterase (AChE) over the human enzyme might be useful in continuing efforts to limit malaria transmission. In this report we assessed 34 synthesized and commercial carbamates for their selectivity to inhibit the AChEs found in carbamate-susceptible (G3) and carbamate-resistant (Akron) An. gambiae, relative to human AChE. Excellent correspondence is seen between inhibition potencies measured with carbamate-susceptible mosquito homogenate and purified recombinant wild-type (WT) An. gambiae AChE (AgAChE). Similarly, excellent correspondence is seen between inhibition potencies measured with carbamate-resistant mosquito homogenate and purified recombinant G119S AgAChE, consistent with our earlier finding that the Akron strain carries the G119S mutation. Although high (100- to 500-fold) WT An. gambiae vs human selectivity is observed for several compounds, none of the carbamates tested potently inhibits the G119S mutant enzyme. Finally, we describe a predictive model for WT An. gambiae tarsal contact toxicity of the carbamates that relies on inhibition potency, molecular volume, and polar surface area. [Copyright &y& Elsevier]

Abstract: To identify potential human-safe insecticides against the malaria mosquito we undertook an investigation of the structure–activity relationship of aryl methylcarbamates inhibitors of acetylcholinesterase (AChE). Compounds bearing a β-branched 2-alkoxy or 2-thioalkyl group were found to possess good selectivity for inhibition of Anopheles gambiae AChE over human AChE; up to 530-fold selectivity was achieved with carbamate 11d. A 3D QSAR model is presented that is reasonably consistent with log inhibition selectivity of 34 carbamates. Toxicity of these compounds to live Anopheles gambiae was demonstrated using both tarsal contact (filter paper) and topical application protocols. [Copyright &y& Elsevier]

Abstract: Anopheles gambiae is the major mosquito vector of malaria in sub-Saharan Africa. At present, insecticide-treated nets (ITNs) impregnated with pyrethroid insecticides are widely used in malaria-endemic regions to reduce infection; however the emergence of pyrethroid-resistant mosquitoes has significantly reduced the effectiveness of the pyrethroid ITNs. An acetylcholinesterase (AChE) inhibitor that is potent for An. gambiae but weakly potent for the human enzyme could potentially be safely deployed on a new class of ITNs. In this paper we provide a preliminary pharmacological characterization of An. gambiae AChE, discuss structural features of An. gambiae and human AChE that could lead to selective inhibition, and describe compounds with 130-fold selectivity for inhibition of An. gambiae AChE relative to human AChE. [Copyright &y& Elsevier]

New insecticides are urgently needed for the control of arthropod vectors of public health diseases. As resistance to many insecticides used for the control of public health pests is ubiquitous, all available chemistries should be evaluated for their potential to effectively control both insecticide-susceptible and insecticide-resistant strains of mosquitoes. This study aimed to evaluate p - p '-difluoro-diphenyl-trichloroethane (DFDT) as a mosquito control technology and relate its activity to that of DDT. We found that topical DFDT was significantly less toxic than DDT to both pyrethroid-susceptible and pyrethroid-resistant strains of Anopheles gambiae and Aedes aegypti. Direct nervous system recording from Drosophila melanogaster CNS demonstrated that DFDT is approximately 10-times less potent than DDT at blocking nerve firing, which may explain its relatively lower toxicity. DFDT was shown to be at least 4500 times more vapor-active than DDT, with an LC 50 in a vapor toxicity screening assay of 2.2 μg/cm2. Resistance to DFDT was assessed in two mosquito strains that possess target-site mutations in the voltage-gated sodium channel and upregulated metabolic activity. Resistance ratios for Akdr (An. gambiae) and Puerto Rico (Ae. aegypti) strains were 9.2 and 12.2, respectively. Overall, this study demonstrates that DFDT is unlikely to be a viable public health vector control insecticide. Unlabelled Image • DFDT is less toxic than DDT topically applied to Aedes aegypti and Anopheles gambiae mosquitoes • Significant resistance was observed for DDT and DFDT on pyrethroid-resistant strains of Aedes aegypti and Anopheles gambiae • DFDT was considerably less effective at producing nerve block in Drosophila melanogaster central nervous system recordings • DFDT is more vapor toxic than DDT and may explain its relatively higher speed of action observed by other researchers. [ABSTRACT FROM AUTHOR]